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Handle async DB calls

I did a couple of projects with node.js and I'm aware of the async behaviour and that one should usually use callback functions, etc. But one thing that bothers me ist the following.
I'm developing an Alexa skill and I have a function that handles the User intent:
'MyFunction': function() {
var toSay = ""; // Holds info what Alexa says
// Lot of checks and calculations what needs to be said by Alexa (nothing special)
if(xyz) {
toSay = "XYZ";
}else if(abc) {
toSay = "ABC";
}else{
toSay = "Something";
}
// Here is the "tricky" party
if(someSpecialEvent) {
toSay += " "+askDatabaseForInput(); // Add some information from database to string
}
this.emit(':ask', toSay, this.t('REPROMT_SPEECH')); // Gives the Info to Alexa (code execution stops here)
}
As mentioned in the code, there is some code which is usually used to find out what the output to Alexa should be.
Only on rare events, "someSpecialEvent", I need to query the database and add information to the String "toSay".
Querying the DB would look something like:
function askDatabaseForInput() { // The function to query the DB
var params = {
TableName: "MyTable",
OtherValues: "..."
};
// Do the Query
docClient.query(params, function(err, data) {
// Of course here are some checks if everything worked, etc.
var item = data.Items[0];
return item; // Item SHOULD be returned
});
return infoFromDocClient; // Which is, of course not possible
}
Now I know, that in the first function "'MyFunction'" I could just pass the variable "toSay" down to the DB Function and then to the DB Query and if everything is fine, I would do the "this.emit()" in the DB Query function. But for me, this looks very dirty and not much reusable.
So is there a way I can use "askDatabaseForInput()" to return DB information and just add it to a String? This means making the asynchronous call synchronous.
Making a synchronous call wouldn't affect the user experience, as the code isn't doing anything else anyway and it just creates the String and is (maybe) waiting for DB input.
Thanks for any help.

So you could do 2 things:
Like the person who commented says you could use a callback:
function askDatabaseForInput(callback) {
var params = {
TableName: "MyTable",
OtherValues: "..."
};
docClient.query(params, function(err, data) {
if (err) {
callback(err, null)
} else {
var item = data.Items[0];
callback(null, item);
}
});
}
or you could use promises:
function askDatabaseForInput() {
var params = {
TableName: "MyTable",
OtherValues: "..."
};
return new Promise(function (resolve, reject) {
docClient.query(params, function(err, data) {
if (err) {
reject(err)
} else {
var item = data.Items[0];
resolve(item);
}
});
});
}
you can then either put a function in where you call askDatabaseForInput or do askDatabaseForInput.then(....).
In the function or the .then you would add what you retrieved from the database to the variable toSay
hope this helps

Cucumber Js callback issue? or feature issue?

I'd like to write a feature like this:
Scenario: new Singleton create
When a new, unmatchable identity is received
Then a new tin record should be created
And a new bronze record should be created
And a new gold record should be created
which would tie to steps like this:
defineSupportCode(function ({ Before, Given, Then, When }) {
var expect = require('chai').expect;
var chanceGenerator = require('./helpers/chanceGenerator')
var request = require('./helpers/requestGenerator')
let identMap;
// reset identMap before each scenario
Before(function () {
identMap = [];
});
// should generate a valid identity
// persist it in a local variable so it can be tested in later steps
// and persist to the db via public endpoint
When('a new, unmatchable identity is received', function (callback) {
identMap.push(chanceGenerator.identity());
request.pubPostIdentity(identMap[identMap.length-1], callback);
});
// use the local variable to retrieve Tin that was persisted
// validate the tin persisted all the props that it should have
Then('a new tin record should be created', function (callback) {
request.pubGetIdentity(identMap[identMap.length-1], callback);
// var self = this;
// request.pubGetIdentity(identMap[identMap.length-1], callback, () => {
// console.log('never gets here...');
// self.callback();
// callback();
// });
// request.pubGetIdentity(identMap[identMap.length-1], (callback) => {
// console.log('never gets here...');
// self.callback();
// callback();
// });
});
The issue that I'm having is that I can't do anything in the Then callback. That is where I'd like to be able to verify the response has the right data.
Here are relevant excerpts from the helper files:
var pubPostIdentity = function (ident, callback) {
console.log('pubIdentity');
var options = {
method: 'POST',
url: 'http://cucumber.utu.ai:4020/identity/' + ident.platform + '/' + ident.platformId,
headers: {
'X-Consumer-Custom-Id': ident.botId + '_' + ident.botId
},
body: JSON.stringify(ident)
};
console.log('ident: ', ident);
request(options, (err, response, body) => {
if (err) {
console.log('pubPostIdentity: ', err);
callback(err);
}
console.log('pubPostIdentity: ', response.statusCode);
callback();
});
}
// accept an identity and retrieve from staging via identity public endpoint
var pubGetIdentity = function (ident, callback) {
console.log('pubGetIdentity');
var options = {
method: 'GET',
url: 'http://cucumber.utu.ai:4020/identity/' + ident.platform + '/' + ident.platformId,
headers: {
'X-Consumer-Custom-Id': ident.botId + '_' + ident.botId
}
};
request(options, (err, response) => {
if (err) {
console.log('pubGetIdentity: ', err);
callback(err);
}
console.log('pubGetIdentity: ', response.body);
callback();
});
}
Something that we are considering as an option is to re-write the feature to fit a different step definition structure. If we re-wrote the feature like this:
Scenario: new Singleton create
When a new, unmatchable 'TIN_RECORD' is received
Then the Identity Record should be created successfully
When the Identity Record is retreived for 'tin'
Then a new 'tin' should be created
When the Identity Record is retreived for 'bronze'
Then a new 'bronze' should be created
When the Identity Record is retreived for 'gold'
Then a new 'gold' should be created
I believe it bypasses the instep callback issue we are wrestling with, but I really hate the breakdown of the feature. It makes the feature less readable and comprehensible to the business.
So... my question, the summary feature presented first, is it written wrong? Am I trying to get step definitions to do something that they shouldn't? Or is my lack of Js skills shining bright, and this should be very doable, I'm just screwing up the callbacks?

Firstly, I'd say your rewritten feature is wrong. You should never go back in the progression Given, When, Then. You are going back from the Then to the When, which is wrong.
Given is used for setting up preconditions. When is used for the actual test. Then is used for the assertions. Each scenario should be a single test, so should have very few When clauses. If you want, you can use Scenario Outlines to mix several very similar tests together.
In this case, is recommend to take it back to first principles and see if that works. Then build up slowly to get out working.
I suspect in this case that the problem is in some exception being thrown that isn't handled. You could try rewriting it to use promises instead, which will then be rejected on error. That gives better error reporting.

Passing csvtojson converter's value to a variable outside of its scope

I'm currently fiddling around with Node.js and I stuck with this issue.
I'm using the csvtojson converter (https://github.com/Keyang/node-csvtojson) as a separate module that I can call in my other JS files as many times as I want.
Here is my tools.js:
module.exports = {
csvToJson: function (csvPath) {
var Converter = require('csvtojson').Converter;
var converter = new Converter({});
var transfer = "DEFAULT";
converter.fromFile(csvPath, function(err, result){
if (err) {
return console.log(err);
}
else {
transfer = result;
}
});
return transfer;
}
};
And here is how I call it:
var countriesCsvFile = path.join(__dirname, '..', 'testDataFiles', 'countries.csv');
//GRAB TOOLS
var tools = require('../app/tools');
console.log(tools.csvToJson(countriesCsvFile));
The result is always the "DEFAULT" value which indicates, that the converter is not touching it.
I want to pass it as the return value of the function, to further be able to process the data on the fly, without creating a file, and read that.
It is surely some scope issue, but after scratching my scalp for a few hours, and browsing the questions I couldn't retrieve anything remotely useful.
Also, another note: If I call console.log(result) instead of transfer = result, it shows me my precious and desired data.

You have to pass in a callback function because the csvToJson function is returning 'transfer' before any value is assigned to it. Like Sirko said, it's asynchronous. You can also use promises instead of callbacks but that's another topic in itself.
module.exports = {
csvToJson: function (csvPath, callback) {
var Converter = require('csvtojson').Converter;
var converter = new Converter({});
converter.fromFile(csvPath, function(err, result){
if (err) {
callback(err);
}
else {
callback(null, result);
}
});
}
};

Functions that can't execute together

I was recently building a scraper module to get some information with nodejs until I encountered this "little" problem. The modules that I'm using are cheeriojs and request.
Actually the module works like a charm if I call only one method at a time. It contains three function and only two of them are exported, this is the code:
'use strict';
var request = require('request'),
cheerio = require('cheerio'),
counter = 0;
function find(term, cat, callback) {
// All the check for the parameters
scrape("http://.../search.php?search=" + encodeURIComponent(term), cat, callback);
}
function last(cat, callback) {
// All the check for the parameters
scrape("http://google.com/", cat, callback);
}
function scrape(url, cat, callback) {
request(url, function (error, response, body) {
if (!error && response.statusCode == 200) {
var $ = cheerio.load(body);
var result = [];
var items = $('.foo, .foo2').filter(function() {
// Condition to filter the resulted items
});
items.each(function(i, row) {
// Had to do another request inside here to scrape other information
request( $(".newpagelink").attr("href"), function(error, response, body) {
var name = $(".selector").text(),
surname = $(".selector2").text(),
link = cheerio.load(body)('.magnet').attr('href'); // This is the only thing that I'm scraping from the new page, the rest comes from the other "cheerio.load"
// Push an object in the array
result.push( { "name": name, "surname": surname, "link": link } );
// To check when the async requests are ended
counter++;
if(counter == items.length-1) {
callback(null, result);
}
});
});
}
});
}
exports.find = find;
exports.last = last;
The problem now, as I was saying, is that if I create a new node script "test.js" and I call only last OR find, it works perfectly! But if I call both the methods consecutively like this:
var mod = require("../index-tmp.js");
mod.find("bla", "blabla", function(err, data) {
if (err) throw err;
console.log(data.length + " find");
});
mod.last(function(err, data) {
console.log(data.length + " last");
});
The results are completely messed up, sometimes the script doesn't even print something, other times print the result of only "find" or "last", and other times returns a cheeriojs error (I won't add here to not mess you up, because probably it's my script's fault). I thought also to repeat the same function two times for both the methods but nothing, the same problems occur... I don't know what else to try, I hope you'll tell me the cause of this behavior!

Your counter variable is global, not specific to each scrape call. It wouldn't work if you called find twice at the same time either, or last.
Move the declaration and initialisation of var counter = 0; into the scrape function, or even better right next to the result and items declarations.

From scanning your code quickly, this is probably due to the variable counter being global. These are asynchronous functions, so they will both act on counter at the same thing. Move the declaration inside of the scrape function.
If you need more information about asynchronous programming, refer to Felix's great answer in this question.

How to use "q" module for refactoring mongoose code?

I'm using mongoose to insert some data into mongodb. The code looks like:
var mongoose = require('mongoose');
mongoose.connect('mongo://localhost/test');
var conn = mongoose.connection;
// insert users
conn.collection('users').insert([{/*user1*/},{/*user2*/}], function(err, docs) {
var user1 = docs[0], user2 = docs[1];
// insert channels
conn.collection('channels').insert([{userId:user1._id},{userId:user2._id}], function(err, docs) {
var channel1 = docs[0], channel2 = docs[1];
// insert articles
conn.collection('articles').insert([{userId:user1._id,channelId:channel1._id},{}], function(err, docs) {
var article1 = docs[0], article2 = docs[1];
}
});
};
You can see there are a lot of nested callbacks there, so I'm trying to use q to refactor it.
I hope the code will look like:
Q.fcall(step1)
.then(step2)
.then(step3)
.then(step4)
.then(function (value4) {
// Do something with value4
}, function (error) {
// Handle any error from step1 through step4
})
.end();
But I don't know how to do it.

You'll want to use Q.nfcall, documented in the README and the Wiki. All Mongoose methods are Node-style. I'll also use .spread instead of manually destructuring .then.
var mongoose = require('mongoose');
mongoose.connect('mongo://localhost/test');
var conn = mongoose.connection;
var users = conn.collection('users');
var channels = conn.collection('channels');
var articles = conn.collection('articles');
function getInsertedArticles() {
return Q.nfcall(users.insert.bind(users), [{/*user1*/},{/*user2*/}]).spread(function (user1, user2) {
return Q.nfcall(channels.insert.bind(channels), [{userId:user1._id},{userId:user2._id}]).spread(function (channel1, channel2) {
return Q.nfcall(articles.insert.bind(articles), [{userId:user1._id,channelId:channel1._id},{}]);
});
})
}
getInsertedArticles()
.spread(function (article1, article2) {
// you only get here if all three of the above steps succeeded
})
.fail(function (error) {
// you get here if any of the above three steps failed
}
);
In practice, you will rarely want to use .spread, since you usually are inserting an array that you don't know the size of. In that case the code can look more like this (here I also illustrate Q.nbind).
To compare with the original one is not quite fair, because your original has no error handling. A corrected Node-style version of the original would be like so:
var mongoose = require('mongoose');
mongoose.connect('mongo://localhost/test');
var conn = mongoose.connection;
function getInsertedArticles(cb) {
// insert users
conn.collection('users').insert([{/*user1*/},{/*user2*/}], function(err, docs) {
if (err) {
cb(err);
return;
}
var user1 = docs[0], user2 = docs[1];
// insert channels
conn.collection('channels').insert([{userId:user1._id},{userId:user2._id}], function(err, docs) {
if (err) {
cb(err);
return;
}
var channel1 = docs[0], channel2 = docs[1];
// insert articles
conn.collection('articles').insert([{userId:user1._id,channelId:channel1._id},{}], function(err, docs) {
if (err) {
cb(err);
return;
}
var article1 = docs[0], article2 = docs[1];
cb(null, [article1, article2]);
}
});
};
}
getInsertedArticles(function (err, articles) {
if (err) {
// you get here if any of the three steps failed.
// `articles` is `undefined`.
} else {
// you get here if all three succeeded.
// `err` is null.
}
});

With alternative deferred promise implementation, you may do it as following:
var mongoose = require('mongoose');
mongoose.connect('mongo://localhost/test');
var conn = mongoose.connection;
// Setup 'pinsert', promise version of 'insert' method
var promisify = require('deferred').promisify
mongoose.Collection.prototype.pinsert = promisify(mongoose.Collection.prototype.insert);
var user1, user2;
// insert users
conn.collection('users').pinsert([{/*user1*/},{/*user2*/}])
// insert channels
.then(function (users) {
user1 = users[0]; user2 = users[1];
return conn.collection('channels').pinsert([{userId:user1._id},{userId:user2._id}]);
})
// insert articles
.match(function (channel1, channel2) {
return conn.collection('articles').pinsert([{userId:user1._id,channelId:channel1._id},{}]);
})
.done(function (articles) {
// Do something with articles
}, function (err) {
// Handle any error that might have occurred on the way
});

Considering Model.save instead of Collection.insert (quite the same in our case).
You don't need to use Q, you can wrap yourself the save method and return directly a Mongoose Promise.
First create an utility method to wrap the save function, that's not very clean but something like:
//Utility function (put it in a better place)
var saveInPromise = function (model) {
var promise = new mongoose.Promise();
model.save(function (err, result) {
promise.resolve(err, result);
});
return promise;
}
Then you can use it instead of save to chain your promises
var User = mongoose.model('User');
var Channel = mongoose.model('Channel');
var Article = mongoose.model('Article');
//Step 1
var user = new User({data: 'value'});
saveInPromise(user).then(function () {
//Step 2
var channel = new Channel({user: user.id})
return saveInPromise(channel);
}).then(function (channel) {
//Step 3
var article = new Article({channel: channel.id})
return saveInPromise(article);
}, function (err) {
//A single place to handle your errors
});
I guess that's the kind of simplicity we are looking for.. right? Of course the utility function can be implemented with better integration with Mongoose.
Let me know what you think about that.
By the way there is an issue about that exact problem in the Mongoose Github:
Add 'promise' return value to model save operation
I hope it's gonna be solved soon. I think it takes some times because they are thinking of switching from mpromise to Q: See here and then here.

Two years later, this question just popped up in my RSS client ...
Things have moved on somewhat since May 2012 and we might choose to solve this one in a different way now. More specifically, the Javascript community has become "reduce-aware" since the decision to include Array.prototype.reduce (and other Array methods) in ECMAScript5. Array.prototype.reduce was always (and still is) available as a polyfill but was little appreciated by many of us at that time. Those who were running ahead of the curve may demur on this point, of course.
The problem posed in the question appears to be formulaic, with rules as follows :
The objects in the array passed as the first param to conn.collection(table).insert() build as follows (where N corresponds to the object's index in an array):
[ {}, ... ]
[ {userId:userN._id}, ... ]
[ {userId:userN._id, channelId:channelN._id}, ... ]
table names (in order) are : users, channels, articles.
the corresopnding object properties are : user, channel, article (ie the table names without the pluralizing 's').
A general pattern from this article by Taoofcode) for making asynchronous call in series is :
function workMyCollection(arr) {
return arr.reduce(function(promise, item) {
return promise.then(function(result) {
return doSomethingAsyncWithResult(item, result);
});
}, q());
}
With quite light adaptation, this pattern can be made to orchestrate the required sequencing :
function cascadeInsert(tables, n) {
/*
/* tables: array of unpluralisd table names
/* n: number of users to insert.
/* returns promise of completion|error
*/
var ids = []; // this outer array is available to the inner functions (to be read and written to).
for(var i=0; i<n; i++) { ids.push({}); } //initialize the ids array with n plain objects.
return tables.reduce(function (promise, t) {
return promise.then(function (docs) {
for(var i=0; i<ids.length; i++) {
if(!docs[i]) throw (new Error(t + ": returned documents list does not match the request"));//or simply `continue;` to be error tolerant (if acceptable server-side).
ids[i][t+'Id'] = docs[i]._id; //progressively add properties to the `ids` objects
}
return insert(ids, t + 's');
});
}, Q());
}
Lastly, here's the promise-returning worker function, insert() :
function insert(ids, t) {
/*
/* ids: array of plain objects with properties as defined by the rules
/* t: table name.
/* returns promise of docs
*/
var dfrd = Q.defer();
conn.collection(t).insert(ids, function(err, docs) {
(err) ? dfrd.reject(err) : dfrd.resolve(docs);
});
return dfrd.promise;
}
Thus, you can specify as parameters passed to cascadeInsert, the actual table/property names and the number of users to insert.
cascadeInsert( ['user', 'channel', 'article'], 2 ).then(function () {
// you get here if everything was successful
}).catch(function (err) {
// you get here if anything failed
});
This works nicely because the tables in the question all have regular plurals (user => users, channel => channels). If any of them was irregular (eg stimulus => stimuli, child => children), then we would need to rethink - (and probably implement a lookup hash). In any case, the adaptation would be fairly trivial.

Today we have mongoose-q as well. A plugin to mongoose that gives you stuff like execQ and saveQ which return Q promises.